1. Field of the Invention
The present invention relates to a technique of generating an image to be displayed on the basis of three-dimensional data.
2. Description of the Background Art
As a method of visualizing (imaging) and displaying data indicative of a three-dimensional object (volume data), a method called volume rendering is known.
The volume rendering includes a method called ray casting. In the ray casting, each time a plane of projection is set, for example, a map (depth map) indicative of a distribution of distances between the plane of projection and a three-dimensional object is generated. By performing sampling along the sight line, volume data is imaged.
By performing various computations by hardware to image volume data at high speed, a problem occurs such that the size of the hardware increases. To address the problem, a technique capable of imaging larger volume data at high speed by hardware of limited size has been proposed (for example, Japanese Patent Application Laid-Open No. 2000-348195).
In the volume rendering, however, each time the direction of sight line, that is, the plane of projection changes, for example, a depth map is re-generated and re-imaging has to be performed, so that a larger amount of computation is necessary. Consequently, some time is necessary to switch visual images in accordance with a change in the direction of sight line. In particular, when the amount of volume data is large, it is difficult to switch images smoothly.
The ray casting includes so-called object order rendering and image order rendering (for example, Japanese Patent Application Laid-Open No. 2002-183748).
The volume rendering is often used in a scene of visualizing volume data of a three-dimensional object such as an internal organ on a screen at a medical site. At the time of observing an affected area or the like while properly changing the direction of sight line, it is desired to switch a display image quickly so as not to apply stress on the user.
For this purpose, various methods of performing volume rendering at higher speed have been devised and proposed. Among them, ray casting of a type (hybrid type) is known in which each time the plane of projection is set, an object order process for generating a depth map indicative of a distribution of distances between the plane of projection and a three-dimensional object is executed, and an image order process of performing sampling along each of sight lines by using the generated depth map is executed, thereby visualizing volume data.
In the conventional ray casting of the hybrid type, generation precision of a depth map is fixed. For example, when the generation precision of a depth map is fixed to be high, there is a tendency that a computation amount of the object order process is relatively large, and a computation amount of the image order process is relatively small. On the other hand, when the generation precision of a depth map is set to be low, there is a tendency that the computation amount of the object order process becomes relatively small, and the computation amount of the image order process becomes relatively large.
However, a problem occurs such that the depth map is generated too roughly or too finely in order to suppress computation time of the image order process, depending on the shape of a three-dimensional object, the entire process time required for the volume rendering increases excessively. In short, the balance between the generation precision of a depth map and the time required to generate an image to be displayed has not been considered much until now. It takes long time to generate an image to be displayed and to switch a display image, and it is difficult to smoothly change a display image in accordance with the switch of the plane of projection, that is, the direction of sight line.